Summer at the Lake is divided into seven major sections, six of which bear titles that mark the progress of summer. Sometimes the dates are called by their secular names, sometimes by names that mark a Catholic feast, and sometimes by both their secular and sacred designations. Epigraphs at the beginning of each segment express the thoughts of various writers about summer and also represent a cross-section of the sacred and the secular.
Within the major divisions of "Prologue," "Memorial Day/Pentecost," "St. John's Night," "Fourth of July," "Mary's Day in Harvest Time," "Labor Day," and "The Feast of St. Michael, Gabriel, Raphael, and all the other Angels," the narratives are again divided between a focus on the year 1948 (the summer when the world as the main characters knew it began to disintegrate) and 1978, the year of the half century of their births. In "Fourth of July," the time jumps to other years as well. Like a fireworks display, this section makes a total of eleven bursts in time and adds further exposition to the historical, social, and economic changes that seared the main characters' lives.
Within this division by years, there is yet another subdivision. The story is told by three separate narrators—Leo Kelly, Fr. Packy Keenan, and Jane Devlin Clare. Sometimes their narratives are interior monologues. Sometimes they are first person narrators of a scenario. Jane's narratives switch halfway through the novel from first-person memoirs to third person omniscient point of view, reflecting her growing ability as a writer to fictionalize the past and distance herself and others from it.